feat(ui): Phase B — sidebar drawer + SystemStatusDot + 키보드 nav

- +layout.svelte: 햄버거 → IconButton, 우측 nav → Button ghost,
  sidebar overlay → Drawer (uiState 단일 slot),
  Esc 글로벌 핸들러 ui.handleEscape() 위임 (5대 원칙 #2)
- lib/stores/system.ts (신규): dashboardSummary writable + 60s 폴링,
  단일 fetch를 SystemStatusDot(B)와 dashboard(C)가 공유
- SystemStatusDot.svelte (신규): 8px 도트 + tooltip,
  failed > 0 → error / pending > 10 → warning / 그 외 → success
- Sidebar.svelte: 트리에 ArrowUp/Down 키보드 nav,
  활성 도메인 row에 aria-current="page"

frontend/src/lib/components/Sidebar.svelte

@@ -66,6 +66,24 @@
   });
 
   let totalCount = $derived(tree.reduce((sum, n) => sum + n.count, 0));
+
+  // ArrowUp/Down 키보드 nav — 현재 펼쳐진 tree-row만 traverse
+  function handleTreeKeydown(e) {
+    if (e.key !== 'ArrowDown' && e.key !== 'ArrowUp') return;
+    const root = e.currentTarget;
+    const rows = Array.from(root.querySelectorAll('[data-tree-row]'));
+    if (rows.length === 0) return;
+    const active = document.activeElement;
+    const idx = active ? rows.indexOf(active) : -1;
+    let next;
+    if (e.key === 'ArrowDown') {
+      next = idx < 0 ? 0 : Math.min(idx + 1, rows.length - 1);
+    } else {
+      next = idx <= 0 ? 0 : idx - 1;
+    }
+    e.preventDefault();
+    rows[next].focus();
+  }
 </script>
 
 <aside class="h-full flex flex-col bg-sidebar border-r border-default overflow-y-auto">
@@ -91,7 +109,7 @@
   </div>
 
   <!-- 트리 -->
-  <nav class="flex-1 px-2 py-2">
+  <nav class="flex-1 px-2 py-2" onkeydown={handleTreeKeydown}>
     {#if loading}
       {#each Array(5) as _}
         <div class="h-8 bg-surface rounded-md animate-pulse mx-1 mb-1"></div>
@@ -123,6 +141,8 @@
 
             <button
               onclick={() => navigate(n.path)}
+              data-tree-row
+              aria-current={isActive ? 'page' : undefined}
               class="flex-1 flex items-center justify-between px-2 py-1.5 rounded-md text-sm transition-colors
                 {isActive ? 'bg-accent/15 text-accent' : isParent ? 'text-text' : 'text-dim hover:bg-surface hover:text-text'}"
             >